UNFCCC (United Nations Framework Convention on Climate Change) enters into force from:

The UNFCCC entered into force on 21 March 1994.

Important PointsUNFCCC (United Nations Framework Convention on Climate Change):

• The United Nations Framework Convention on Climate Change (UNFCCC) established an international environmental treaty to combat "dangerous human interference with the climate system", in part by stabilizing greenhouse gas concentrations in the atmosphere.
• It was signed by 154 states at the United Nations Conference on Environment and Development (UNCED), informally known as the Earth Summit, held in Rio de Janeiro from 3 to 14 June 1992.
• It established a Secretariat headquartered in Bonn and entered into force on 21 March 1994
• The UNFCCC secretariat (UN Climate Change) is the United Nations entity tasked with supporting the global response to the threat of climate change.  
• UNFCCC stands for United Nations Framework Convention on Climate Change.
• The Convention has near-universal membership (197 Parties) and is the parent treaty of the 2015 Paris Agreement.
• The main aim of the Paris Agreement is to keep the global average temperature rise this century as close as possible to 1.5 degrees Celsius above pre-industrial levels.
• The UNFCCC is also the parent treaty of the 1997 Kyoto Protocol.
• The ultimate objective of all three agreements under the UNFCCC is to stabilize greenhouse gas concentrations in the atmosphere at a level that will prevent dangerous human interference with the climate system, in a time frame that allows ecosystems to adapt naturally and enables sustainable development.

Hence, the correct answer is 21 March 1994